(a) Field of the Invention
The present invention relates to a method for refining 2,6-naphthalene dicarboxylic acid, and more particularly, to a method for refining 2,6-naphthalene dicarboxylic acid capable of obtaining a product having excellent purity and color in an economical and environmentally friendly manner.
(b) Description of the Related Art
Polyesters produced by the polymerization of 2,6-naphthalene dicarboxylic acid with a diol are known to excel in several properties such as thermal stability, tensile strength, gas permeability, etc., and as such are chosen as good materials for films, fibers, storage containers, etc. In particular, polyethylene naphthalate (PEN) produced by the polymerization of 2,6-naphthalene dicarboxylic acid and ethylene glycol is expected to replace polyethylene terephthalate (PET).
2,6-Naphthalene dicarboxylic acid is obtained by oxidizing 2,6-dimethyl naphthalene with oxygen gas in the presence of catalytic cobalt, manganese, and bromine compounds. The thus-obtained crude 2,6-naphthalene dicarboxylic acid contains numerous impurities including acids having one functional group such as formyl naphthoic acid, methyl naphthoic acid, etc. that are generated by incomplete oxidation of 2,6-dimethyl naphthalene; trimellitic acid that is obtained from the collapse of naphthalene structure; and brominated naphthalene dicarboxylic acid, naphthoic acid, naphthalene tricarboxylic acid, colored organic impurities whose structures are not identified, and metal impurities such as cobalt complex, manganese complex, etc.
Polyesters obtained by the polymerization of ethylene glycol and crude 2,6-naphthalene dicarboxylic acid containing impurities as mentioned above show poor physical properties, heat stability, structural stability, and so on. Moreover, such polyesters are classified as low quality because they are tinged with color.
Of the impurities, mono carboxylic acids such as methyl naphthoic acid, naphthoic acid, etc. are especially problematic. If these mono carboxylic acids exceed a certain amount, the polymerization rate is decreased during the production of polyesters, and gelation and coloring occur. In particular, formyl naphthoic acid is especially problematic. Accordingly, so as to obtain polyesters having high quality, it is important to reduce these impurities.
2,6-Naphthalene dicarboxylic acid cannot be refined by distillation because it is degraded at high temperature, and it is not easy to refine it by re-crystallization because it is not dissolved well in general solvents.
Until now, several refinement methods of 2,6-naphthalene dicarboxylic acid have been known. The first method is to dissolve 2,6-naphthalene dicarboxylic acid in general solvents and then recrystallize it. The second method is to convert 2,6-naphthalene dicarboxylic acid into its alkali salt and then dissolve and recrystallize it. The third method is to convert 2,6-naphthalene dicarboxylic acid into its amine salt and then dissolve and recrystallize it. Also, industrially, pure dimethyl 2,6-naphthalene dicarboxylate is produced by reacting 2,6-naphthalene dicarboxylic acid with methanol to prepare dimethyl 2,6-naphthalene dicarboxylate (2,6-NDC), and then refining it through distillation to be sold. However, as can be seen in the preparation of polyethylene terephthalate resins, as raw materials for the synthesis of polyesters, acids are superior to esters in processing and economics, and therefore studies on methods capable of directly refining 2,6-naphthalene dicarboxylic acid in a convenient and economical manner are required.
As a known method for refining 2,6-naphthalene dicarboxylic acid by dissolving it in general solvents and then recrystallizing it, U.S. Pat. No. 5,256,817 discloses a method for refining 2,6-naphthalene dicarboxylic acid by dissolving it in water or acetic acid and then hydrogenating and crystallizing it. However, as this method requires heating to a high temperature in order to dissolve the 2,6-naphthalene dicarboxylic acid, the production cost of naphthoic acid is increased and it also requires expensive metal catalysts for hydrogenation, and is thus problematic.
Japanese Patent Publication No. 62-230747 A discloses a method of dissolving 2,6-naphthalene dicarboxylic acid in polar solvents such as dimethylsulfoxide, dimethylformamide, dimethylacetamide, etc., adsorbing it to activated carbons, hydrogenating it, and then crystallizing it. However, such method requires a quantity of solvents and activated carbons, the solvents may be hydrogenated, and the formyl naphthoic acid is not eliminated and thus the yield of the products is low. Japanese Patent Publication No. 5-32586 A discloses a method for refining 2,6-naphthalene dicarboxylic acid by dissolving it using pyridine or pyridine derivatives as a solvent and then crystallizing it, but this method is also problematic because the solubility of 2,6-naphthalene dicarboxylic acid in the solvents is not sensitive to temperature and its yield is thus low.
As another method for refining 2,6-naphthalene dicarboxylic acid, there is a method of converting it into its metal salt and then dissolving and recrystallizing it. Japanese Patent Publication No. 52-20993 A and Japanese Patent Publication No. 48-68544 B disclose a method for refining 2,6-naphthalene dicarboxylic acid by dissolving it in a KOH or NaOH aqueous solution to thereby prepare its alkali metal salt, then adsorbing it with a solid adsorbent and crystallizing it. Disproportionation of the thus-produced mono alkali salt with water generates a refined 2,6-naphthalene dicarboxylic acid. However, this method requires a quantity of solid adsorbents and solvents, and as all mono alkali salts are crystallized, salts generated by impurities such as naphthoic acid or formyl naphthoic acid, etc. are also crystallized and it is difficult to separate them.
Japanese Patent Publication No. 52-20994 B and Japanese Patent Publication No. 48-68555 B disclose a method for refining 2,6-naphthalene dicarboxylic acid using a diacid salt. This method is carried out by dissolving 2,6-naphthalene dicarboxylic acid in a KOH or NaOH aqueous solution, then adsorbing it to an adsorbent and crystallizing the produced diacid salt. However, this method also requires a large amount of adsorbents, it is difficult to eliminate a small amount of alkali salt impurities, and it has a low yield. Japanese Patent Publication No. 2-243652 A discloses a method of dissolving 2,6-naphthalene dicarboxylic acid in an alkali aqueous solution and then precipitating an alkali salt by adding a polar organic solvent that is well mixed with water thereto, but when 2,6-naphthalene dicarboxylic acid having high purity is obtained, it has the problem that its yield is low.
There are proposed methods of dissolving 2,6-naphthalene dicarboxylic acid into the form of its amine salt and then recrystallizing it. The produced 2,6-naphthalene dicarboxylic acid in the form of an amine salt is heated to a temperature above the boiling point of the amine to thereby generate pure 2,6-naphthalene dicarboxylic acid. Japanese Patent Publication No. 50-142542 A discloses a method of dissolving 2,6-naphthalene dicarboxylic acid in an amine aqueous solution and then precipitating 2,6-naphthalene dicarboxylic acid in the form of an amine salt by distilling and concentrating the solvent. Japanese Patent Publication No. 50-135062 A discloses a method of dissolving 2,6-naphthalene dicarboxylic acid in an amine aqueous solution and then precipitating it by cooling or condensing the solution, and Japanese Patent Publication No. 5-294892 A discloses a method of obtaining pure 2,6-naphthalene dicarboxylic acid by dissolving 2,6-naphthalene dicarboxylic acid in a mixed solution of an alcohol and an amine, then precipitating it with an amine salt and heating it. However, these methods have the problem that when 2,6-naphthalene dicarboxylic acid having a high purity is obtained, its yield is low.
U.S. Pat. No. 5,859,294 discloses a method of obtaining pure 2,6-naphthalene dicarboxylic acid by mixing 2,6-naphthalene dicarboxylic acid with an amine, dissolving it in a mixed solution of water, a ketone, or acetonitrile, cooling it to thereby precipitate the diamine salt of 2,6-naphthalene dicarboxylic acid, and then distilling it. However, this method has the problem of requiring a high temperature and high pressure to obtain 2,6-naphthalene dicarboxylic acid with a high yield.
Therefore, study on methods for refining 2,6-naphthalene dicarboxylic acid capable of obtaining 2,6-naphthalene dicarboxylic acid having excellent purity and color in a convenient and economical manner are urgently required.